1. Field of the Invention
The present invention relates to a hydraulic control apparatus for work machines, and more particularly relates to a hydraulic control apparatus used in a work machine equipped with a dozing blade that has left and right hydraulic cylinders used for tilting, such as bulldozers and the like.
2. Description of the Related Art
(Conventional Art 1)
FIG. 2 shows the peripheral parts of the blade installed on the front part of the vehicle body of a bulldozer in a perspective view.
Bulldozers perform work such as digging and transporting earth, and leveling the ground surface following such excavation by means of a blade 3 (dozing blade) that is attached to the front part of the vehicle main body.
A pair of tilting cylinders, i. e., left and right tilting cylinders 4 and 5, are installed between the blade 3 and vehicle main body.
If both of the tilting cylinders 4 and 5 are simultaneously driven in the same direction (in extension or retraction), the blade 3 is placed in a pitch dump attitude (forward-inclined attitude) or pitch back attitude (rearward-inclined attitude).
Furthermore, if one of the tilting cylinders is placed in stopped state, and the other tilting cylinder is driven in extension or retraction, the blade 3 assumes an attitude in which the right end part or left end part of the blade 3 is tilted downward (right-tilted attitude or left-tilted attitude). This is called a single tilting operation. The performance of a single tilting operation is described in U.S. Pat. No. 5,799,737.
Furthermore, if one of the tilting cylinders is driven in extension or retraction at the same time that the other tilting cylinder is driven in extension or retraction, the operation speed of the tilting operation of the blade 3 is increased. This is called a dual tilting operation. The performance of a dual tilting operation is described in U.S. Pat. No. 4,802,537 and U.S. Pat. No. 6,481,506.
(Conventional Art 2)
FIG. 5A shows the hydraulic circuit in a case where two fixed displacement hydraulic pumps 105 and 104 are used as a pressurize oil supply source for the left and right tilting cylinders 103 and 102.
As is shown in FIG. 5A, left and right tilting cylinders 103 and 102 are attached to the blade 101. Fixed displacement hydraulic pumps 105 and 104 are installed corresponding to the left and right tilting cylinders 103 and 102; furthermore, main operating valves 107 and 106 in which the direction and flow rate of the pressurized oil are controlled are installed respectively corresponding to the left and right cylinders 103 and 102.
The pressurized oil that is discharged from the fixed displacement hydraulic pump 105 is supplied to the bottom end chamber 103B or head end oil chamber 103H of the left tilting cylinder 103 via the main operating valve 107. Similarly, the pressurized oil that is discharged from the fixed displacement hydraulic pump 104 is supplied to the bottom end chamber 102B or head end chamber 102H of the right tilting cylinder 102 via the main operating valve 106.
(Conventional Art 3)
FIG. 6 shows the hydraulic circuit 110 in a case where a single variable displacement type hydraulic pump 111 is used as the pressurized oil supply source of the left and right tilting cylinders 103 and 102.
In order to prevent the construction of the hydraulic circuit from becoming complicated, the main operating valves 107 and 105 are connected in parallel to a single variable displacement hydraulic pump 111.
Specifically, as is shown in FIG. 6, left and right tilting cylinders 103 and 102 are attached to the blade 101. Main operating valves 107 and 106 in which the direction and flow rate of the pressurized oil are controlled are installed corresponding to the left and right tilting cylinders 103 and 102. The discharge port of the variable displacement hydraulic pump 111 is caused to communicate with the inlet port of the main operating valve 107 via a pressure compensating valve 113, and is caused to communicate with the inlet port of the main operating valve 106 via a pressure compensating valve 112.
If the system is devised so that left and right tilting cylinders 103 and 102 are simultaneously driven by the single variable displacement hydraulic pump 111 without pressure compensating valves 113 and 112, even if the opening areas of the main operating valves 107 and 106 are varied by the same amount by operating the operating levers, a large flow rate will be supplied on the side of the tilting cylinder with a smaller load (e. g., the left tilting cylinder 103), and only a small flow rate will be supplied on the side of the tilting cylinder with a larger load (e. g., the tilting cylinder 102).
Accordingly, pressure compensating valves 113 and 112 are installed for the respective main operating valves 107 and 106 so that flow rates corresponding to the amounts of operation of the operating levers are supplied to the left and right tilting cylinders 103 and 102 without being affected by the load.
Hydraulic pressure compensation is accomplished by the installation of the pressure compensating valves 113 and 112. As a result, the differential pressure before and after the constriction on the side with a light load, e. g., the main operating valve 107, is the same value as the differential pressure before and after the constriction of the main operating valve 106 on the side with a heavy load.
As a result of pressure compensation thus being performed, the differential pressures before and after the constrictions of both main operating valves 107 and 106 are the same value so that flow rates proportional to the degrees of opening of the main operating valves 107 and 106, i. e., proportional to the amounts of operation of the operating levers, are supplied to the tilting cylinders 103 and 102 without being affected by the load.